By N. Rasul. Henderson State Univerisity. 2018.
In addi- In the absence of an evidence-based practice guideline tion to the well-recognized risk for pulmonary com- for the perioperative management of an older person at plications purchase finasteride 5 mg without prescription, such as signiﬁcant lung disease and type and increased risk of pulmonary complications finasteride 1mg discount, a pragmatic duration of operation, a patient’s functional level should approach is required. Patients with better exercise tol- encourage abstinence from cigarettes, eradicate tracheo- erance by self-report or by the measured distance during bronchial infections, relieve airﬂow obstruction, and a timed walk have fewer pulmonary complications than instruct the patient in lung expansion maneuvers. Postoperatively, deep-breathing or angina, location and length of the incision, and ASA exercise and incentive spirometry should be encouraged, class of 3 or greater. It has been estimated that between quantitative ventilation–perfusion scan can accurately 20% and 30% of patients undergoing general surgery predict postoperative ﬂow rates; when the predicted develop deep venous thrombosis, and the incidence is as postoperative forced expiratory volume in 1 s is 0. Although fatal pulmonary embolism function tests for patients undergoing abdominal pro- 65,66 occurs in 1% to 5% of all surgical patients, it accounts for cedures is unproven. Additional studies are needed a larger proportion of operative deaths in middle-aged to address whether pulmonary function test results con- 75 and older individuals. Because venous thrombosis and tribute signiﬁcant additional information to what is known pulmonary emboli can be difﬁcult to diagnose and treat, from the clinical examination and whether this informa- considerable effort has been focused on prophylaxis. Various regimens to reduce deep venous thrombosis and Prospective studies in high-risk patients have identiﬁed pulmonary emboli, including heparin, warfarin, aspirin, speciﬁc interventions that reduce morbidity and mortal- dextran, and leg compressive devices, have been used. Stein and Cassara found The recommendations for prevention of venous throm- that "poor-risk" patients treated with a regimen of pre- boembolism of the Sixth American College of Chest operative smoking cessation, antibiotics "when indi- Physicians Consensus Conference on Antithrombotic cated," perioperative bronchodilator therapy, inhalation Therapy related to older surgical patients are summa- of humidiﬁed gas, postural drainage, and chest physio- 76 rized in Table 20. Clinical risk factors identiﬁed by therapy had fewer pulmonary complications, lower this group include: increasing age; prolonged immobility, mortality, and shorter hospital stays than nontreated 67 stroke or paralysis; previous venous thromboembolism; patients. Data on the impact of cigarette smoking cancer and its treatment; obesity; varicose veins; suggest that as much as 6 weeks of abstinence may be cardiac dysfunction; indwelling central venous catheters; required before there is improvement in small airways inﬂammatory bowel disease; nephrotic syndrome; and disease, hypersecretion of mucus, tracheobronchial clear- estrogen use. Clinical setting Recommended prophylaxis General surgery >40 years old, nonmajor surgery, no other risk factors LDUH, LMWH, ES or IPC >40 years old, major surgery or >60 years old, nonmajor surgery, with other risk factors LDUH, LMWH, or IPC Higher risk patients with a greater than usual risk of bleeding ES or IPC Very high risk patients with multiple risk factors LDUH or LMWH combined with ES or IPC Orthopaedic surgery Hip replacement LMWH or adjusted dose warfarin (INR 2-3) Knee replacement LMWH or adjusted dose warfarin (INR 2-3) Hip fracture repair LMWH or adjusted dose warfarin (INR 2-3) Neurosurgery Intracranial operations IPC with or without ES LDUH, low dose unfractionated heparin; LMWH, low molecular weight heparin; ES, elastic stockings; IPC, intermittent pneumatic compression Source: Geerts et al. The urine sodium is is a loss of renal mass, primarily in the cortex, that results generally greater than 40 mEq/L, and the urine to plasma in a 30% to 50% decrease in the number of glomeruli by creatinine ratio is generally less than 10 : 1. Management of acute intrinsic generally coincident with a decline in muscle mass so that renal failure includes discontinuing potentially nephro- the serum creatinine levels may remain normal. Even with appropriate treatment, acute equation proposed by Cockcroft and Gault78: postoperative renal failure has a mortality rate between 40% and 80%. With aging, the loss of functioning nephrons been shown to correlate with measured creatinine clear- increases the solute load per nephron, and renal blood ance in older patients. Older individuals also have a diminished the risk of postoperative acute renal failure. Renal blood thirst perception that compromises their ability to ﬂow can be compromised intraoperatively because of a respond to signiﬁcant free water loss and hyperosmolal- ity. These factors and nephrotoxic volume overload sometimes results from the delayed medications can result in postoperative acute reversible response to sodium restriction and salt wasting observed 81 in older individuals,84 which can be exacerbated by the intrinsic renal failure. Oliguria, isosthenuria, and a rising serum creatinine are early clinical signs of this syndrome. Sodium and water retention after surgery may last In all diabetic patients undergoing surgery, it is impor- for several days. Values should pounded by the difﬁculties in the clinical assessment of be obtained preoperatively, during the procedure, and in volume status in older persons. Afterward, the frequency of moni- an important sign of intravascular volume depletion but toring will be determined by the treatment regimen, the can be observed in euvolemic older patients and may be patient’s condition, and glucose control. For patients difﬁcult to assess properly in the immediate postopera- whose blood sugar can be maintained in the normal range tive period. When the assessment of volume status by diet and exercise therapy, no special preoperative becomes critically important, it is often necessary to preparation is required. Hyperglycemia can be effec- measure pulmonary capillary wedge pressure using a tively treated with supplemental short-acting insulin Swan–Ganz catheter. The patients receiv- Intravenous ﬂuid administration must be adjusted ing oral hypoglycemic medications should have these for the older surgical patient because there is a decline held on the day of the operation. Hyperglycemia can be in both total body water and intracellular water with treated with short-acting insulin. For men between 65 and 85 years of age insulin, several management regimens are possible. More commonly, for lular volume is approximately 20% to 25% of body patients normally treated with a single dose of insulin weight. In the absence of acute stress and conditions each day, one-half to two-thirds of the usual dose of known to affect salt and water balance, the daily meta- insulin is given on the morning of surgery, and a glucose- bolic requirements per liter of intracellular ﬂuid are as containing intravenous solution is administered at a rate follows: of 5 to 10 g glucose per hour. For patients who are normally managed with multiple Water, 100 mL does of insulin throughout the day, one-third the usual Energy, 100 kcal morning dose is administered on the morning of sur- Protein, 3 g gery, and a glucose-containing solution is infused intra- Sodium, 3 mmol 89 venously. Blood sugar control is easier if a constant rate Potassium, 2 mmol of infusion of the glucose solution is maintained while For example, an 80-year-old woman weighing 40 kg has nonglucose-containing intravenous ﬂuids are used to an estimated intracellular volume of 10 L. Additional nance requirements would be 1 L water, 1000 kcal, 30 g doses of regular insulin should be administered to control protein, 30 mmol sodium, and 20 mmol potassium. Fluid blood sugar levels; a 6-h interval between glucose meas- and electrolyte status must be closely monitored and urements is commonly used. In addition to meticulous adjusted according to the response of the patient and the attention to blood sugar levels, it is important to monitor development of other pathophysiologic conditions. Myocardial ischemia can be silent and Endocrine Disorders may be detected unexpectedly on postoperative electro- Diabetes mellitus, usually type II, is common among cardiograms. It has been estimated that of diabetic Thyroid disease is not as prevalent as diabetes but, if patients undergoing surgery, more than 75% are over the undetected, can result in major complications periopera- age of 50. The prevalence of hypothyroidism in hospitalized ment of surgical patients but also predisposes the patient older patients has been reported to be 9. With the stress and tissue injury of tain a high index of suspicion for thyroid illness in this surgery, there is an increase in many of the counterregu- population.
Considering how the diagnosis is going to affect your current job and future career is therefore a matter of considerable importance purchase 5 mg finasteride. Many issues arise in this context buy cheap finasteride 1 mg, including how or indeed whether to tell your employer; whether it might still be possible to continue work and, if so, on what basis, and what the implications might be ﬁnancially. Telling your employer You need to think this situation through beforehand, and rehearse what you might say. It is very important that your employer knows something about MS before you speak to them if possible. Any negative, or less than positive, reaction to what you say may be due as much to ignorance of MS, as to any particular problem with you personally. The MS Society has produced a helpful leaﬂet called Employing People with Multiple Sclerosis – some questions answered, and this would be worth giving or sending to your employer. When you are talking to your employer, you must remember that, in terms of his or her response to you, they are thinking in business terms, however much they might like you personally. So it is important that you understand this and present in effect a ‘business case’ to them. A business case would emphasize your training, experience, commitment and your value to the organization, and would present a realistic – and thus modest – view of the likely problems that you might ‘cost’ the organization in terms of absence for sickness in the foreseeable future. It would also indicate that your abilities in many areas of your work were unlikely to be affected. If there are minor changes in working practices or additional equipment that you might need, not just to compensate for reduced mobility, for example, but also your productivity and hence your organization’s enhancement, then try arguing for them. For many 145 146 MANAGING YOUR MULTIPLE SCLEROSIS employers, keeping skilled personnel who know the organization and its objectives and clients is more preferable to ﬁnding new employees, especially if they are convinced that you will continue to perform well in your job. Whilst the battle to reduce prejudice against people with MS at work is gradually being won for people in their current jobs, many employers still have a concern about promoting people with the condition. In your current job you will have proved yourself, that is almost certainly why you are applying for promotion, and thus your employer – presumably – is likely to be satisﬁed with your work in that job. However, he or she may ﬁnd the combination of your promotion to a new position where you will not yet, of course, have demonstrated your competence, and a condition with variable symptoms, difﬁcult to be positive about. So, if you tell your employer about your MS, you should stress the qualities that you have (unaffected by the MS), and how important these would be for the job you are going after. It may well be that your previous skills and experience are such that a positive decision on promotion is relatively easy. Telling your colleagues Given the way that news gets around, it is unlikely that you will be able to tell one colleague without others becoming aware of your situation quite quickly. Despite your wishes, sometimes it can even happen that information from outside your work situation alerts colleagues about your MS unintentionally, for example an inadvertent message from a family member to a colleague about an absence from work. So it is probably wise to work out ways in which to tell your colleagues in a planned process. Although most of your colleagues will have probably heard something about MS, their views will be based on a wide range of experiences and ideas, and thus may not be accurate. The best thing may be to give each of your colleagues some written information about MS – perhaps one or more of the pamphlets on MS available from the MS Society – at the time you are informing them about your situation. They can then have EMPLOYMENT 147 accurate information, and you can respond to any questions that they might want to ask you about your own MS. It may be worth reminding them, if they were not aware of your MS before you told them, that this shows how little your work, and your working relationships with them were affected – and indeed this may continue for a long time. The Disability Discrimination Act 1995 and employment The provisions of the Disability Discrimination Act 1995 are in principle very substantial, and apply to many aspects of employment. However, the exact implications of many of the provisions have not yet all been legally tested, so it will only become clear over the years how precisely the Act will apply. It is important to remember that the Act applies to organizations and companies with over 20 employees, although those with under this number are expected to abide by the spirit of the provisions. Broadly, the position under the Act is that unlawful discrimination in employment occurs in the following circumstances: • when a disabled person is treated less favourably than someone else; • this treatment is given for a reason relating to that person’s disability; • the reason does not apply to the other person, and • the treatment cannot be justiﬁed. Such discrimination must not occur in: • the recruitment and retention of employees; • promotion and transfers; training and development, and • the dismissal process. In addition employers must make reasonable changes to their premises or employment arrangements if these substantially dis- advantage a disabled employee, or prospective employee, compared to a non-disabled person. These provisions sound formidable and very supportive of the situation of many people with MS, and in many respects they may be; however, the detailed interpretation of the provisions of the Act awaits clariﬁcation. Many of the provisions of the Act hinge on what a ‘substantial’ disadvantage to a disabled person is, and what is ‘a 148 MANAGING YOUR MULTIPLE SCLEROSIS reasonable’ adjustment on the employer’s part is. Nevertheless, some examples may help to clarify certain provisions: • Employers probably cannot justify dismissing disabled employees if they were sometimes off work because of their disability, if the amount of time they take off is what the employers accept as sick leave for other employees. Examples of changes to physical features that may be required are: • widening doorways; • changing taps to make them easier to turn; • altering lighting for people with restricted vision, and • allocating a particular parking space for a disabled person’s car. Examples of changes to procedures or practices that may be required are: EMPLOYMENT 149 • altering working hours; • supplying additional training; • allocating some duties to another employee; • allowing absences during working hours for rehabilitation, assessment and treatment; • providing a reader or interpreter; • providing supervision; • acquiring or making changes to equipment; • modifying procedures for testing or assessment, or • transferring person to another place of work. Further information on the provisions of the Act can be obtained from the Disability Discrimination Act Information Line (see Appendix 1). There is also a booklet containing guidance and a code of practice on employment available from the Stationery Ofﬁce (see Appendix 2). Exceptions to the Act Although all permanent, temporary and contract workers are covered, certain organizations or work settings are not covered. These include: • people in the armed services; • police ofﬁcers; • ﬁre brigade members if they are expected to take part in ﬁreﬁghting; • Ministry of Defence ﬁreﬁghters • prison ofﬁcers and prison custody ofﬁcers; • people working on board a ship, aircraft or a hovercraft; • people who work outside the UK; • individual franchise holders with less than 20 employees, even if the whole franchise network has more than 20. As a different kind of exception, there are charities and organizations providing supported employment who can discriminate in favour of disabled people.
Virtual visualizations of the trachea discount finasteride 1mg mastercard, esophagus 1 mg finasteride otc, and colon have been com- pared to standard endoscopic views by endoscopists, who judged them to be 1. Quantitative measurements of geometric and densitometric information obtained from the VE images (virtual biopsy) are being carried out and compared to direct measures of the original data. Preliminary analysis suggests that VE can provide accurate and reproducible visualizations. Such studies help drive improvements in and lend credibility to VE as a clinical tool. Panel A is a transparent rendering of a portion of the large bowel selected for segmenta- Figure 1. Volume renderings of anatomic structures segmented from a spiral CT of patient with colon cancer. Panels Band C, reveal the same anatomic segment from di¨erent viewpoints after the skin has been removed. Note the magni®ed view of the rectal tumor (panel E); a polyp is also identi®ed, segmented, and rendered in the mid-sigmiodal region. Panel A is a texture-mapped view of the polyp at close range, and panel B shows an enhancement of the polyp against the luminal wall. This type of enhancement is possible only with VE, because the polyp can be digitally segmented and processed as a separate object. Panel C is a transparent rendering of the polyp revealing a dense interior region, most likely a denser-than-normal vascular bed. Both geo- metric and densitometric measures may be obtained from the segmented data. Before powering the ablation electrode, the electrical activity on the inner surface of the heart chamber must be painstakingly mapped with sensing electrodes to locate the anomaly. To create the map with a single conventional sensing electrode, the cardiologist must manipulate the electrode via the catheter to a point of interest on the chamber wall by means of cine- 1. Only after the position of the sensing electrode on the heart wall has been unambiguously identi®ed may the signal from the electrode be analyzed (primarily for the point in the heart cycle at which the signal arrives) and mapped onto a representation of the heart wall. Sensed signals from several dozen locations are needed to create a useful representation of cardiac electrophysiology, each requiring signi®cant time and e¨ort to unambiguously locate and map. The position and extent of the anomaly are immediately obvious when the activation map is visually com- pared to normal physiology. After careful positioning of the ablation electrode, the ablation takes only a few seconds. The morbidity associated with this procedure is primarily related to the time required (several hours) and complications associated with extensive arterial catheterization and repeated ¯uoroscopy. There is signi®cant promise for de- creasing the time for and improving the accuracy of the localization of sensing electrodes by automated analysis of real-time intracatheter or transesophageal ultrasound images. Any methodology that can signi®cantly reduce procedure time will reduce associated morbidity; and the improved accuracy of the map- ping should lead to more precise ablation and an improved rate of success. My group is developing a system wherein a static surface model of the target heart chamber is continuously updated from the real-time image stream. A gated 2-D image from an intracatheter, transesophageal, or even hand-held transducer is ®rst spatially registered into its proper position relative to the heart model. The approximate location of the sectional image may be found by spatially tracking the transducer or by assuming it moved very little from its last calculated position. More accurate positional information may be derived by surface-matching contours derived from the image to the 3-D surface of the chamber (36). As patient-speci®c data are accumulated, the static model is locally deformed to better match the real-time data stream while retaining the global shape features that de®ne the chamber. Once an individual image has been localized relative to the cardiac anatomy, any electrodes in the image may be easily referenced to the correct position on the chamber model, and data from that electrode can be accumulated into the electrophysiologic mapping. To minimize the need to move sensing electrodes from place to place in the chamber, Mayo cardiologists have developed ``basket electrodes,' or multi-electrode packages that deploy up to 64 bipolar electrodes on ®ve to eight ¯exible splines that expand to place the electrodes in contact with the chamber wall when released from their sheathing catheter (37). The unique geometry of these baskets make the approximate positions of the elec- trodes easy to identify in registered 2-D images that capture simple landmarks from the basket. Cardiac electrophysiology displayed on left ventricle viewed from (A) outside and (B) inside the left ventricle. Most of the techniques are used for the management of pain and include deep nerve regional anesthesiology procedures. The process of resident training involves a detailed study of the anatomy associated with the nerve plexus to be anesthesitzed, including cadavaric studies and practice needle insertions in cadavers. Because images in anatomy books are 2-D, only when the resident examines a cadaver do the 3-D anatomic relationships become clear. In addi- tion, practice needle insertions are costly because of the use of cadavers and limited by the lack of physiology. To address these issues, my group has been developing an anesthesiology training system in our laboratory in close coop- eration with anesthesiology clinicians (38). A variety of anatomic structures were identi®ed and segmented from CT and cryosection datasets. The segmented structures were subsequently tiled to create models used as the basis of the training system. Because the system was designed with the patient in mind, it is not limited to using the Visible Human Anatomy.
He introduced the idea of interrelationships between the various systems buy finasteride 5 mg online, an equilibrium whereby one system affects the in- tegrity of another (for example 1mg finasteride fast delivery, one’s vision cannot be perfect if one’s hearing is defective). In this way he defined 14 principal muscles and 28 additional muscles, and established a system of how they were con- nected, founded on the use of neuro-vascular points, neuro-lymphatic points, and on the scanning of the meridian lines. Denisson created educational kinesiology, or edukine- siology, by stretching the concepts of right brain and left brain — which are major weapons in the theoretical arsenal of New Age medi- cine, even though they have no real anatomical-physiological reality. It is true that in right-handed individuals, the left hemisphere is dominant and is used mainly for written and spoken language; how- ever, it is also well known that a person with a cranial trauma, in which some of the left hemisphere functioning has been lost, may be rehabili- tated to some degree by "reactivating" the identical structures in the uninjured right hemisphere. By contrast, no clinical experiment has proven that the left brain governs reason and the right brain emotions, as so many trendy techniques suggest — any more than there is one brain for conscious and one brain for unconscious or subconscious 7 processing. Taking up Goodheart’s concept of energy flows, from a new angle, 71 Healing or Stealing? Jimmy Scott developed a theory that old or recent, physical or psychic obstructions of the energy flow influence our rela- tionship to the environment and predetermine certain pathologies. Thus, he posited that allergies exist because of blocked energy, caused when the subject is confronted with a parasitic energy whose vibra- tions are not in harmony with his primordial energy, or that establishes resonance with the blocked energy zone (! W hiteside, Callaway and Stokes then came up with the one brain/one health concept, and began working on the emotional causes of psychic and physical disorders, which they felt could be corrected by de-energizing these causes in the past and by liberating the system of conditioned beliefs. They invented the concept of harmonic kinesiology (three-in-one concept), or integrated brain. Diamond’s behavioral kinesiology would integrate the influ- ence of the environment on the individual (agressology), his way of life (ethology and ethnology), and nutrition (diet), together with the effect of positive and negative thoughts on the individual’s energy level. Bruce Dewe and his wife, Joan, developed Integral Health (Professional Kinesiology Practice) in New Zealand, and expanded the use of energy balancing. Alain Beardall introduced the concept of the digital de- terminator, and finally Dr. Verity (a good name) created the blue print series that was intended to eliminate the negative ego and to find the origin of our fears — the negative ego being responsible for our diseases and pains, our codependencies and the various inherited beliefs and habits that underlie our repetitive behaviors. Principles of Kinesiology Kinesiology uses simple and precise muscular tests to examine the body and identify the nature, the location, the intensity, the history, and the origin of energy blockages so that the therapist can adapt a pro- gram of exercises to correct them. Using simple muscular tests, we can test a person to find out how he is organized, what are his dominant tendencies, how the communication is organized be- tween brain and eye, brain and ear, brain and hand, etc.. W e can bet- ter understand where the blockages or hold-ups occur, and how we can remedy them. It is these blockages that usually cause the difficul- ties we encounter at various stages of education, whatever our age. They also contribute to our constant stress, to difficulties of concen- tration and of communication, and they can even create muscular tensions that lead to poor posture. One might say that the body car- ries in itself the means of doing away with these blockages; using the appropriate tests, KINESIOLOGY can interrogate the body, and thus can understand and read the answers that the body itself offers for the problems encountered. W hen we give the body the neces- sary means to clear up these blockages, we very quickly see a clear improvement in everything that relates to the simplest activities such as reading, writing, seeing, hearing, remembering. Thus, kinesiologists believe that by probing the muscles with appropriate tests it would be possible to tap into this memory and the blockages that it generates. Let’s take a look at some excerpts from an advertising brochure put out by a group on edukinesiology. The two cerebral hemispheres are connected by a kind of bridge named the "corpus callosum", a complex bundle of nervous fibers that allows communication and coordination between these two parts of the brain. If, for any reason, this connection does not function cor- rectly, or if it is interrupted, the person will present very serious dis- orders that will handicap his general functioning. The right brain governs the "reflexes"; it perceives the overall picture in a given situation. It enables us to recognize a melody from the first two notes, or to recognize faces in a crowd. The left brain is "analytical"; it breaks up information into minimal units and deals with it sequentially. It controls the right part of the body, and is much emphasized in our education system, for it is the hemi- sphere of logic, which our. Neither hemisphere holds priority over the other but, quite to the contrary, complementary functioning is the rule, and it is precisely the lack of speedy connections between the two that lies at the origin of slow development in learning, expression, communication. W hen we talk about predominance, in educational kinesiology, it is in the context of looking to find out which of the two hemispheres the per- son more readily uses, in a given situation, and why he has trouble integrating and using the whole range of possibilities that he has at his disposal. Any secondary school student learns that the reflexes are seated in the spinal cord and not the brain. All the subsequent rea- soning is thus off-base and is re-interpreted in favor of kinesiologic practice. He then uses muscular and gymnastic exercises in an effort to rehabilitate the brain through its muscular connections. This technique has the merit of borrowing from the disciplines of speech therapy, physical therapy, and functional rehabilitation; but it rests on several theoretical inconsistencies, especially in regard to the brain’s role. Furthermore, proponents of this technique present it as the cure to whatever ails you. One brochure suggests that it will elimi- nate problems including: x physical: back pains, joint problems, migraines, eczema, coli- tis, impotence, sterility, ear-eye-nose-throat problems, etc. The positive results obtained at "brain gym" sessions with young children are due solely to the additional attention given to the "problem children". But questions must be asked when, in the context of a sug- gested training curriculum, esoteric concepts crop up that traditionally belong to patamedicine: the law of the five elements, the law of seven dimensions, the seven barometric tests, the four stages of evolution and 75 Healing or Stealing?
Example 2: high prevalence of carotid artery disease Disease No disease (Carotid artery (no carotid disease) artery disease) Total Test positive (positive CTA) 500 10 510 Test negative (negative CTA) 100 120 220 Total 600 130 730 Results: sensitivity = 500/600 = 0 discount 1mg finasteride with amex. Equations for calculating the results in the previous examples are listed in Appendix 1 buy 5mg finasteride mastercard. Qualitative literature summary The keystone of the evidence-based imaging (EBI) approach is to critically assess the research data that are provided and to determine if the infor- mation is appropriate for use in answering the EBI question. Unfortunately, the published studies are often limited by bias, small sample size, and methodological inadequacy. Further, the information provided in pub- lished reports may be insufﬁcient to allow estimation of the quality of the research. Two recent initiatives, the CONSORT (1) and STARD (2), aim to improve the reporting of clinical trials and studies of diagnostic accuracy, respectively. This chapter summarizes the common sources of error and bias in the imaging literature. Random error occurs due to chance variation, causing a sample to be different from the underlying population. Systematic error, or bias, is an incorrect study result due to nonrandom distortion of the data. Using the bull’s-eye analogy, the larger the sample size, the less the random error and the larger the chance of hitting the center of the target. In systematic error, regardless of the sample size, the bias would not allow the researcher to hit the center of the target. A second way to think about random and sys- tematic error is in terms of precision and accuracy (3). The larger the sample size, the more precision in the results and the more likely that two samples from truly different populations will be differentiated from each other. Using the bull’s-eye analogy, the larger the sample size, the less the random error and the larger the chance of hitting the center of the target (Fig. System- atic error, on the other hand, is a distortion in the accuracy of an estimate. Regardless of precision, the underlying estimate is ﬂawed by some aspect of the research procedure. Using the bull’s-eye analogy, in systematic error regardless of the sample size the bias would not allow the researcher to hit the center of the target (Fig. Random error is divided into two main types: Type I, or alpha error, is when the investigator concludes that an effect or difference is present when in fact there is no true difference. Type II, or beta error, occurs when an investigator concludes that there is no effect or no difference when in fact a true difference exists in the underlying population (3). Quantiﬁcation of the likelihood of alpha error is provided by the familiar p value. In effect, the difference observed in a sample is due to chance variation rather than a true underlying difference in the population. Type I Error There are limitations to the ubiquitous p values seen in imaging research reports (4). In other words, there could be a very large difference between two groups under study, but the p value might not be signiﬁcant if the sample sizes are small. Conversely, there could be a very small, clinically unimportant difference between two groups of subjects or between two Chapter 2 Critically Assessing the Literature 21 imaging tests, but with a large enough sample size even this clinically unimportant result would be statistically signiﬁcant. Because of these limitations, many journals are underemphasizing the use of p values and encouraging research results to be reported by way of conﬁdence intervals. Conﬁdence Intervals Conﬁdence intervals are preferred because they provide much more infor- mation than p values. Conﬁdence intervals provide information about the precision of an estimate (how wide are the conﬁdence intervals), the size of an estimate (magnitude of the conﬁdence intervals), and the statistical signiﬁcance of an estimate (whether the intervals include the null) (5). If you assume that your sample was randomly selected from some pop- ulation (that follows a normal distribution), you can be 95% certain that the conﬁdence interval (CI) includes the population mean. More precisely, if you generate many 95% CIs from many data sets, you can expect that the CI will include the true population mean in 95% of the cases and not include the true mean value in the other 5% (4). Whereas the p value is often interpreted as being either statistically signiﬁcant or not, the CI, by providing a range of values, allows the reader to interpret the implications of the results at either end (6,7). In addition, while p values have no units, CIs are presented in the units of the variable of interest, which helps readers to interpret the results. The CIs shift the interpretation from a qualitative judgment about the role of chance to a quantitative estimation of the biologic measure of effect (4,6,7). As an example, two hypothetical transcranial circle of Willis vascular ultrasound studies in patients with sickle cell disease describe mean peak systolic velocities of 200cm/sec associated with 70% of vascular diameter stenosis and higher risk of stroke. However, the narrower conﬁdence intervals for the larger study reﬂect the greater precision, and indicate the value of the larger sample size. For a smaller sample: 50 95 CI 200 1 96( ) 50 95% CI 200 14 186 214 For a larger sample: 50 95 CI 200 1 96( ) 500 22 C. Type II Error The familiar p value does not provide information as to the probability of a type II or beta error. The size of the sample studied may be too small to detect an important difference even if such a difference does exist. The ability of a study to detect an important difference, if that difference does in fact exist in the underlying population, is called the power of a study. Power analysis can be performed in advance of a research investigation to avoid type II error. Power Analysis Power analysis plays an important role in determining what an adequate sample size is, so that meaningful results can be obtained (8).
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